Food moves along the digestive tract as the result of contractions by smooth muscle.
Smooth muscle is an involuntary non-striated muscle, so named because it lacks sarcomere and hence striations (ligaments or bands).
It is divided into he two subgroups of simple and multipartite smooth muscles. Within a unitary muscle, whole bundles or sheets of smooth muscle cells contract as syncytia.
Smooth muscle is found in the walls of hollow organs such as the stomach, intestines, bladder, and uterus. In the walls of passages such as blood vessels and lymph vessels, and within the ducts of the respiratory, urinary, and reproductive systems.
In the eye, the ciliary muscle, a type of smooth muscle, expands and contracts the iris and changes the shape of the lens. In the skin, smooth muscle cells, such as those in the pili muscle, make the hair stand up in response to cold or fear.
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